Conventional electrical telephone speech networks are all of a parallel type, which means that the bias currents for the transmitting amplifier and the receiving amplifier are independent. Accordingly, the minimum working current for a parallel speech network is typically twice that of a serial type. The line current supplied to a speech network decreases as the speech network gets further from the exchange. Therefore, a parallel type speech network is more difficult to operate properly than a serial type at locations remote from the exchange, owing to lower line current. If the minimum working current of a speech network is higher, the number of extension telephones that can be connected to in parallel is decreased.
Besides, a mute circuit in a conventional electrical telephone speech network usually exhibits malfunction characteristics resulting from bias switching operation. For example, in pulse dialing mode, the mute circuit usually disables operation of a receiving amplifier and causes an increase in loop impedance so as to raise the line voltage drop of the whole network during "make" status. A pulse dialing telephone set utilizes a switch in conjunction with the speech network. By definition, "make" status occurs when the above-mentioned switch is on, a current flows through the telephone set, and the relay in an exchange is turned on. A mute circuit may cause malfunction of the relay in the exchange and it is difficult to meet the requirement of certain industrial standards, for example, EIA RS-470.
While in DTMF dialing mode, the conventional electrical telephone speech network utilizes a different DTMF transmitting amplifier, rather than the speech transmitting amplifier, to transmit signals. According to this conventional implementation, the DTMF transmitting amplifier needs extra working current for dialing so as to reduce the line voltage drop which, decreases the AC signal dynamic range. For this reason, the conventional type speech network cannot produce a larger DTMF signal and can easily cause distortion of signals, owing to the saturation of active devices.
Furthermore, a conventional electrical telephone speech network uses a sidetone cancellation network to acquire the DTMF sidetones. Additional distortion therefore results. Besides, the signal voltage level is very unstable. All the foregoing drawbacks cause the users to experience discomfort in hearing.
For example, FIG. 5 is a circuit block diagram of parallel type of speech network disclosed by M. Faheem Akram et al. in the IEEE Journal of Solid-State Circuits, Vol. SC-18, No. 6, December 1983. In this network, the bias currents of transmitting and a receiving amplifiers are designated I.sub.TX and I.sub.RX respectively and flow through different bias current routes. Further, this network structure acquires DTMF sidetones from a telephone line by a sidetone network.
The objectives of the present invention are to improve the above-mentioned disadvantages in conventional speech networks. This invention utilizes a serial type of circuit architecture to reduce the minimum working current. It also utilizes the auto bias switching method to maintain the most suitable line voltage drop, in order to meet the different requirements of the dialing modes and the talking states. At the same time, the direct coupling method is put into practice to decrease the distortion and control the volume stability of DTMF sidetones.
In accordance with the present invention, a serial type of electrical telephone speech network has power amplifiers of transmitting loop and receiving loop cascaded serially. Since the above-mentioned two power amplifiers utilize the same bias current, the minimum working current of the network is reduced. Further, with the uniquely designed bias switching circuit, in the pulse dialing mode, the line voltage drop of the whole network is automatically decreased during "make" status, that is, the loop impedance is reduced. As a consequence, the relay in the exchange can operate more reliably, so that the exchange can receive more accurate signals. In contrast, while in DTMF dialing mode, the bias switching circuit automatically increases the line voltage drop to provide the DTMF signal with a larger AC dynamic range, so as to decrease the distortion and enable a larger signal to be transmitted further. Furthermore, the directly coupled DTMF sidetone is the least distorted and the most stable in volume that telephone users can hear.